Monoamine oxidase (MAO, E.C.1.4.3.4) is a major enzyme in the metabolism of the amine neurotransmitters and other significant biogenic amines. As such, it has been implicated in a number of neuropsychiatric disorders. There have been recent reports of reduced platelet MAO activity in conditions such as schizophrenia, alcoholism and bipolar depression and the platelet enzyme has been looked to as a possible reflection of the enzyme in the brain. However, there is considerable uncertainty about the relationship between the brain and platelet enzyme, especially in view of substrate specificity differences and the lack of MAO type A in blood platelets. This study proposes to examine the possible correlations between MAO found in blood platelets and MAO in brain tissue from the aspects of microenvironmental influences and protein structure. It has been suggested that the lipid microenvironment of the enzyme in the mitochondrial outer membrane influences its activity. Such influences on brain and platelet MAO activity will be compared through Arrhenius temperature studies. Further direct influence of lipid on MAO activity will be examined in delipidation studies. Through the use of radiolabelled pargyline, labeled MAO subunits will be obtained by gel electrophoresis and subjected to limited proteolysis. A comparison of the resultant peptides from the brain and platelet enzyme will be made for indications of similarities or non similarities.